1. Field of the Invention
The present invention relates to an image display apparatus. More specifically, the present invention relates to an image display apparatus using a liquid crystal display panel displaying an image by controlling a polarization state of linearly polarized light.
2. Description of the Related Art
Liquid crystal display panels used in projection-type image display apparatuses are roughly classified into a transmission type and a reflection type.
Referring to FIG. 6, a conventional projection-type image display apparatus using a transmission-type liquid crystal display panel will be described. The image display apparatus includes an illumination unit having a lamp 22 and a concave mirror 23, a transmission-type liquid crystal display panel 27, an input side polarizing plate 28.sub.i, an output side polarizing plate 28.sub.o, a field lens 29, a projection lens 30, and a screen 31.
A light beam from the illumination unit 21 is modulated by the liquid crystal display panel 27 through the polarizing plate 28.sub.i. Then, the light beam passes through the field lens 29 through the polarizing plate 28.sub.o, and is projected on the screen 31 by the projection lens 30. The liquid crystal display panel 27 is provided with the polarizing plate 28.sub.i and the polarizing plate 28.sub.o as polarization selecting elements on a light beam input side (the left side of the drawing) and a light beam output side (the right side of the drawing) in such a manner that their polarization axes are orthogonal to each other. An image is displayed by modulating the linearly polarized light by the liquid crystal display panel 27 so as to control its transmittance. In the case where an electrical field is not applied to a liquid crystal layer in the liquid crystal display panel 27, light transmitted through the polarizing plate 28.sub.i is rotated (i.e., a plane of polarization is rotated) along a twist of liquid crystal molecules by an optical anisotropic effect of the liquid crystal molecules so as to be capable of passing through the polarizing plate 28.sub.o. Consequently, a bright display state (bright state) can be obtained. In the case where an electrical field is applied to the liquid crystal layer, the twist orientation of the liquid crystal molecules is cancelled, so that the liquid crystal layer does not rotate the plane of polarization. As a result, the light transmitted through the polarizing plate 28.sub.i cannot pass through the polarizing plate 28.sub.o, being blocked by the polarizing plate 28.sub.o. Therefore, a dark display state (dark state) is obtained.
The ratio of brightness between a bright state and a dark state in the image display apparatus with the above-mentioned structure is called a contrast ratio. The contrast ratio is an important factor in an image display quality which largely depends upon a degree of polarization P of the two polarizing plates 28.sub.i and 28.sub.o for the following reason: the amount of transmitted light in the case of displaying a dark state largely depends upon the degree of polarization P of the polarizing plates 28.sub.i and 28.sub.o ; therefore, light is transmitted even in the case of displaying a dark state when the degree of polarization P is low; as a result, the contrast ratio decreases.
According to "Introduction to Optics II" (J. Tsujiuchi, Asakura Shoten, p. 203), the degree of polarization P is defined by Formula (1) using a total intensity I.sub.tot and a complete polarized light component I.sub.pol of a light beam transmitted through a polarizing plate: EQU P=I.sub.pol /I.sub.tot (1)
According to "Polarizing-phase difference film for LCD" listed on pp. 53 to 55 in the August, 1995 issue of Monthly Display (Technotimes), the degree of polarization P of a polarizing plate tends to decrease with the increase in transmittance T of the polarizing plate. Therefore, increasing the transmittance T of the polarizing plate so as to realize a bright display in a liquid crystal display apparatus results in decreasing the degree of polarization P of the polarizing plate. This decreases a contrast ratio in the liquid crystal display apparatus. Thus, the transmittance T and the degree of polarization P of two polarizing plates 28.sub.i and 28.sub.o used in the projection-type image display apparatus using the transmission-type liquid crystal display panel is determined by the tradeoff of the brightness and the contrast ratio of a display. In general, the display quality remarkably decreases at a contrast ratio of less than 100:1, so that the degree of polarization P is required to be at least about 99%. In most cases, the polarization degrees of the polarizing plates 28.sub.i and 28.sub.o are at least about 99.9%.
Next, referring to FIG. 7, a conventional projection-type image display apparatus using a reflection-type liquid crystal display panel will be described. The image display apparatus includes an illumination unit 21 having a lamp 22 and a concave mirror 23, a polarizing plate 28.sub.i, a polarization beam splitter 32, a polarizing plate 28.sub.o, a reflection-type liquid crystal display panel 27', a field lens 29, a projection lens 30, and a screen 31. A light beam from the illumination unit 21 is incident upon the polarization beam splitter 32 through the polarizing plate 28.sub.i. A P-polarized light component of the light beam passes through the polarization beam splitter 32, and an S-polarized light component thereof is reflected in a side direction of the polarization beam splitter 32 to be incident upon the liquid crystal panel 27'. In the liquid crystal display panel 27', the S-polarized component has its plane of polarization optically modulated in the same way as in the above-mentioned transmission-type liquid crystal panel and is reflected. Thus, the P-polarized light component included in the reflected light can pass through the polarization beam splitter 32, and is projected on a screen 31 through the field lens 29 and the projection lens 30, whereby an image is displayed.
In the projection-type image display apparatus with the above-mentioned structure, the degree of polarization of the polarization beam splitter 32 with respect to a light beam reflected therefrom or transmitted therethrough largely influences the contrast ratio. However, even if the polarization beam splitter 32 has a degree of polarization of at least about 99% with respect to incident light (e.g., laser light) whose scattering angle is negligible, in an optical system of the projection-type image display apparatus, illumination light generally has a scattering angle of .+-. several degrees; therefore, the degree of polarization of the polarization beam splitter 32 is lower, compared with those of the polarizing plates 28.sub.i and 28.sub.o. More specifically, since the transmittance of the P-polarized light component with respect to the polarization beam splitter 32 is about 95%, and that of the S-polarized light component is about 5%, the degree of polarization of the polarization beam splitter 32 is very low, i.e., about 95% in accordance with the above-mentioned Formula (1). As a result, in the projection-type image display apparatus with the above-mentioned structure, the contrast ratio is very low, i.e., 20:1. In order to solve this problem, the following attempt has been made: the polarizing plates 28.sub.i and 28.sub.o are provided as polarization selecting elements on an illumination unit side and a projection element side of the polarization beam splitter 32, and the degree of polarization of the combination of the polarizing plate 28.sub.i and the polarization beam splitter 32 and that of the combination of the polarizing plate 28.sub.o and the polarization beam splitter 32 are prescribed to be at least about 99%, whereby a contrast ratio of at least 100:1 is achieved.
However, in the projection-type image display apparatus using the transmission-type liquid crystal display panel as shown in FIG. 6, an effective component of the illumination light which can be used by the transmission-type liquid crystal display panel 27 is limited to that along a polarization axis of the polarizing plate 28.sub.i. The component which cannot pass through the transmission-type liquid crystal display panel 27 is absorbed by the polarizing plate 28.sub.i and converted into heat. Therefore, the use efficiency of light becomes 1/2 or less. As a result, a bright display cannot be obtained in such a projection-type image display apparatus. Furthermore, due to the heat absorption by the polarizing plate 28.sub.i, the characteristics of the polarizing plate 28.sub.i are degraded and the temperature of the liquid crystal layer in the liquid crystal display panel 27 is increased to phase-change a liquid crystal material included therein. Thus, display characteristics are degraded.
Likewise, in the projection-type image display apparatus using the reflection-type liquid crystal display panel as shown in FIG. 7, the polarizing plate 28.sub.i on the illumination unit side of the polarization beam splitter 32 transmits only linearly polarized light along its polarization axis and converts the other component into heat which it absorbs. Therefore, the use efficiency of light becomes 1/2 or less. Furthermore, due to the heat absorption by the polarizing plate 28.sub.i, the characteristics of the polarizing plate 28.sub.i are degraded and the temperature of the liquid crystal layer in the liquid crystal panel 27' is increased to phase-change a liquid crystal material included therein. Thus, display characteristics are degraded.
In order to solve the above-mentioned problems, the following techniques have been proposed. For example, Japanese Laid-open Publication No. 4-178683 describes a liquid crystal display apparatus (projection-type image display apparatus) in which a component of illumination light from an illumination unit which cannot pass through an input side polarizing plate is converted into polarized light which can pass through the input side polarizing plate by a polarization separating element and a polarization rotating element. A polarization converting optical system of the liquid crystal display apparatus described in this publication is shown in FIG. 8. As shown in FIG. 8, the polarization converting optical system includes an illumination unit 41 having a lamp 42 and a concave mirror 43, a cube-shaped polarization beam splitter 44, a polarization rotating element 45, a reflector 46, a prism 47, and a transmission-type liquid crystal display panel 48.
The polarization beam splitter 44 transmits a particular polarized light component (P-polarized light a.sub.1 in FIG. 8) and reflects a polarized component orthogonal thereto (S-polarized light a.sub.2 in FIG. 8) in a side direction of the polarization beam splitter 44, thereby splitting the light beam. Thus, white random light emitted from the illumination unit 41 is split into the P-polarized light a.sub.1 and the S-polarized light a.sub.2 by the polarization beam splitter 44. The P-polarized light a.sub.1 transmitted through the polarization beam splitter 44 is incident upon the prism 47 as it is. The S-polarized light a.sub.2 reflected in the side direction passes through the polarization rotating element 45, thereby having its plane of polarization rotated by 90.degree..
Thus, the plane of polarization of the S-polarized light a.sub.2 is aligned with the same direction as that of the P-polarized light a.sub.1 (i.e., direction parallel to the drawing surface) to become P-polarized light a.sub.2 '. Thereafter, the P-polarized light a.sub.2 ' is reflected by the reflector 46 and is incident upon the prism 47. Then, two light beams of the P-polarized light a.sub.1 and the P-polarized light a.sub.2 ' are combined by the prism 47 and radiated to the transmission-type liquid crystal display panel 48. In this way, according to the structure described in the above-mentioned publication, two light beams of the P-polarized light a.sub.1 and the S-polarized light a.sub.2 are radiated to the transmission-type liquid crystal display panel 48 with their planes of polarization being alighted, whereby the use efficiency of light is improved, and the increase in temperature and the decrease in display characteristics of the transmission-type liquid crystal display panel 48 are minimized.
Japanese Laid-open Publication No. 2-93580 describes a projection-type image display apparatus as shown in FIG. 9. The projection-type image display apparatus includes a reflector 56 on a surface of a polarization beam splitter 55 orthogonal to a transmission-type liquid crystal display panel 57, and a quarter-wave plate 54 which is a polarization rotating element disposed between an illumination unit 51 irradiating a parallel light beam and the polarization beam splitter 55. With this structure, the technique described in the publication contemplates that all the components of the light beam from the illumination unit 51 are radiated to the transmission-type liquid crystal display panel 57. In the optical system shown in FIG. 9, the polarization beam splitter 55 transmits P-polarized light a.sub.1 and reflects S-polarized light a.sub.2 in a side direction, thereby splitting white random light emitted from the illumination unit 51. The reflected S-polarized light a.sub.2 is reflected by the reflector 56 and reflected again by the polarization beam splitter 55. Then, the S-polarized light a.sub.2 travels back to the illumination unit 51, is further reflected by a concave mirror 53 of the illumination unit 51 twice, and is incident upon the polarization beam splitter 55.
During the above-mentioned process, the polarization state of the S-polarized light a.sub.2 changes as follows. When returning to the concave mirror 53 of the illumination unit 51 from the polarization beam splitter 55, the S-polarized light a.sub.2 passes through the quarter-wave plate 54 to become circularly polarized light. Then, the circularly polarized light passes through the quarter-wave plate 54 to become P-polarized light a.sub.2 ', thereby having its plane of polarization aligned with the P-polarized light a.sub.1. Accordingly, the S-polarized light a.sub.2 is re-used, so that the amount of light which can be used by the transmission-type liquid crystal display panel 57 increases, and the increase in temperature of the liquid crystal layer and the decrease in display characteristics can be minimized.
Japanese Laid-open Publication No. 4-127120 describes a polarization converting optical system not using a polarization rotating element. The structure of the optical system is shown in FIG. 10. The optical system includes a polarization beam splitter 64 between an illumination unit 61 irradiating a parallel light beam and a transmission-type liquid crystal display panel 65. The polarization beam splitter 64 transmits P-polarized light a.sub.1 of white random light from the illumination unit 61 and reflects S-polarized light a.sub.2 thereof to the illumination unit 61. The S-polarized light a.sub.2 reflected from the polarization beam splitter 64 is reflected by a concave mirror 63 twice and is incident upon the polarization beam splitter 64. At this time, depending upon the positions at which the S-polarized light a.sub.2 is reflected in a cross-section of the concave mirror 63, linearly polarized light having a different polarization axis (i.e., P-polarized light a.sub.2 ') is generated. Thus, the P-polarized light a.sub.1 and a.sub.2 ' pass through the polarization beam splitter 64 to be radiated to the transmission-type liquid crystal display panel 65. Therefore, the use efficiency of light is improved and the heat generation of the liquid crystal layer and the degradation of the display characteristics are minimized.
However, the above-mentioned techniques have respective problems which will be described below.
According to the technique (FIG. 8) described in Japanese Laid-open Publication No. 4-178683, the liquid crystal display panel 48 is irradiated with light beams from two directions, so that the-incident angle of illumination light becomes large and the optical system itself becomes large. When the incident angle of the illumination light becomes large, the aperture of the projection lens of the projection unit should be enlarged. Furthermore, the number of components such as the polarization beam splitter 44, the polarization rotating element 45, and the like increases. This causes the apparatus to be enlarged and the cost to be increased.
According to the technique (FIG. 9) described in Japanese Laid-open Publication No. 2-93580, the polarization beam splitter 55, the reflector 56, and the quarter-wave plate 54 are required to be provided. Therefore, the technique shown in FIG. 9 also has the problem that the apparatus is enlarged, and the cost is increased in the same way as in the technique described in Japanese Laid-open Publication No. 4-178683. In addition, because of the absorption and reflection of illumination light by the quarter-wave plate 54, the loss of the illumination-light is caused. Furthermore, since the quarter-wave plate 54 is disposed close to the illumination unit 51, the characteristics of the quarter-wave plate 54 are degraded by the heat of the illumination unit 51.
The technique (FIG. 10) described in Japanese Laid-open Publication No. 4-127120 also has the problem that the apparatus is enlarged and the cost is increased because of the presence of the polarization beam splitter 64. Furthermore, a discontinuous portion is present in a polarization separating film of the polarization beam splitter 64. It is very difficult to uniformly form the polarization separating film in the discontinuous portion. This changes the polarization separation characteristics, resulting in display inconsistency.
As described above, there has been a demand for a miniaturized and light-weight projection-type image display apparatus which has outstanding use efficiency of light from an illumination unit, enables a bright image with a high contrast to be obtained, has an outstanding display quality, and is produced at low cost.